Show End Date incorrectly checked depending upon user creating/editing node.

This appears to be a slightly different twist on issues that have cropped up previously.
Using Date module 7.x-2.6 there are problems with 'Show End Date'.
Closely related issue... Problem with Show End Date

For the site administrator things are fine.
On creating a new node, the 'Show End Date' appears as unchecked and only the start date field is visible.
Check the 'Show End Date' and both fields appear and dates can be added to each.

Similarly when the administrator edits nodes things appear normal. 'Show End Date' will be checked or unchecked depending upon whether both start and end dates or only the start date were previously saved.
So for example, as in image provided, previously saved content with an event start and finish dates will appear in the editor with 'Show End Date' checked just as you'd expect.

However, if any user is given permission to create or edit an existing node then the 'bug', which has been discussed at some length in the past, takes effect.
The 'Show End Date' is shown as checked for previously saved start and finish dates.... but only the start date actually appears in the editor. It's necessary to uncheck 'Show End Date' for the end date field to appear with its saved content.

In every instance of either creating or editing content the options available to permitted users are the opposite of what would be thought to be correct.

The image shows the difference when the identical node is presented to the site administrator and to a user who has been given full edit permissions.... Show End Date image

Effectively it means the error kicks in when any user, other than the site administrator, creates or edits the date fields.